A U.S. official ridiculed as "another blatant lie" a Syrian government report Thursday asserting that terrorists -- not security forces -- massacred civilians in Houla.

Last weekend's massacre, which left more than 100 people dead, sparked outrage across the globe and prompted calls for action against the regime of Bashar al-Assad.

But Syria attributed the latest violence to "armed terrorist groups," the vague entities that the regime has blamed all along for widespread violence against civilians during the nearly 15 months of unrest.

"The goal of the armed operation was to completely terminate the presence of the state in the area and to make it one that is out of the control of the state," Qasim Jamal Sleiman, head of the investigative panel, said in televised remarks.

"All of the martyrs are from peaceful families who refused to stand against the state and have never demonstrated or carried weapons against the state. They were in disagreement with the armed terrorist groups, which confirms that there was a goal and an interest to kill them."

The U.S. ambassador to the United Nations, Susan Rice, called the Syrian account "another blatant lie" and said there's no "factual evidence" to "substantiate that rendition of events." She said the U.N. Human Rights Council in Geneva, Switzerland, could soon embark on an effort to establish facts in the case and hold people accountable.

U.S. Defense Secretary Leon Panetta said the international community needs to ensure al-Assad steps down.

"There is no question that we are very concerned about the atrocities that are taking place in Syria," he said. "Just makes clear how important it is to remove Assad from power and to try to implement the necessary political reforms that are necessary in that country."

The Syrian government investigation said 600 to 800 armed people gathered after Friday prayers at two primary locations and committed the crimes. Sleiman said firearms shot from a close distance and sharp objects were used, but there was no shelling.

"The place where the massacre was committed is an area where armed terrorist groups are present," Sleiman said. "The security forces did not enter the area before or after the massacre and the area is far from the checkpoints where the security forces are positioned."

But he said security forces "defended themselves against the armed terrorist groups."

Some of the attackers hailed from the Houla area, investigators said. "Also, some of the bodies that were shown as part of the massacre are bodies of armed individuals who were killed during their attack on the security forces and they are not from the town."

Politicians across the world, opposition leaders and Syrian citizens blame the regime, citing witness accounts that pro-government forces were responsible for the Houla bloodshed. They say government forces have been responsible for violence in Syria since March 2011.

The massacre spurred diplomatic action this week. The United States, Netherlands, Australia, Britain, France, Germany, Italy, Japan, Spain, Bulgaria, Turkey and Canada announced that they are expelling Syrian diplomats.

Rice has said the massacre was carried out by Shabiha militias or local gangs acting on behalf of the regime.

Survivors told Human Rights Watch that the army shelled the area and "armed men, dressed in military clothes, attacked homes on the outskirts of town and executed entire families."

A network of Syrian opposition activists, the Local Coordination Committees of Syria, blamed "armed militias" of the Syrian government.

"This barbaric act was preceded by the regime's mortar shelling in the town," the LCC said in a statement. "The campaign ended when the armed militias slaughtered entire families in cold blood."

Sectarian tensions have been high in Houla, which is overwhelmingly Sunni and is surrounded by Alawite and Shiite villages. The regime is dominated by Alawites.

The government report came as U.S. Secretary of State Hillary Clinton ramped up pressure Thursday on Russia, saying the Kremlin has been an obstacle to peace in Syria.

"I think they are, in effect, propping up the regime at a time when we should be working for transition," Clinton told reporters in Denmark.

The United States and Russia have been looking for solutions to the 15 months of persistent violence. Estimates of the number of dead range from 9,000 to more than 14,000.

The United States is focused on supporting U.N. and Arab League envoy Kofi Annan's six-point peace plan. The administration is hoping Russia can persuade Syrian President Bashar al-Assad to adhere to the plan and keep the country from deteriorating into more warfare.

The Syrian regime said it supported the Annan plan, which includes a cease-fire. But so far, according to the secretary of state, the Syrian regime has failed to abide by the initiative.

"The Russians keep telling us they want to do everything they can to avoid a civil war because they believe the violence will be catastrophic" and they have likened the situation to the "equivalent of a very large Lebanese civil war," Clinton said.